Playing the Juggernaut, which is identical imperial version of the guardian, I disagree completely. Perhaps if you are putting too much into the DPS trees he is weak, but as a tank they are delicious. Course I would no doubt say the story is better on the Imperial version, so there is that. Been doing fine in PVP as well, once you get in your later 20's the Juggernauts/Guardians really get lot of essential abilities that work well for them in pvp. In PVE, they work quite well as well. Only way I can see someone not doing well with them is if they are not going with the tank build, as for dps builds/trees, they just are one of the crappier paths to go since all the other dps build options in the other ACs have much better utility and mechanics.
They are tank classes, and in most cases I think people getting into them don't know what to expect out of playing a tank. DPS and other classes usually are more interesting overall in MMOs. The ranged tanks also are much more flexible in how they can be played, and with all the jedi running around stabbing at each other, the ranged tanks often can stay at a safe distance and just play how they want, as a support, dps, etc, instead of a damage sponge like the Guardian/Juggernaught. Course a tank in pvp, well you are always going to seem at a disadvantage, because you are basically forced to play with other players to support them as your damage is pretty weak. This is why though lot of mmo now allow players to dual spec at least for pvp purposes alone if need be just because the pve class design does not equate to equal/fun pvp play. I would really like them to put it pvp dual specs. They seem resistant to dual specs right now, but they should let us have a PVP build option